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The Genetic Consequences of Inbreeding Depression
Inbreeding Depression Inbreeding occurs when two genetically related organisms produce offspring. This may occur both intentionally and unintentionally in many species around the globe. While in human populations inbreeding is taboo and discouraged, it does occur in society today. Among animal populations, many animals do not differentiate between relative and nonrelative; in some of these populations, inbreeding results in minimal consequences (1; 2). Typically, inbreeding has led to many health consequences among human populations. These issues are caused by the genetic problems associated with producing offspring with someone of a very similar genetic background. When the sperm and egg from two genetically unrelated individuals meet, two very different sets of genetic material are meshed. Meiosis, a process by which chromosomes are recombined and divided, results in a unique recombination of chromosomal DNA. This unique combination is the major difference between an individual and their parents, their siblings, and their other relatives. When the sperm and egg from two genetically related individuals meet, two similar sets of genetic material are meshed. This results in a very similar, albeit slightly different, recombination of chromosomal DNA. While this may not seem to be a major issue, on the genetic level, this may be very lethal to the inbred individual. Typically, recessive, undesirable alleles are eliminated from a gene pool over many generations of sexual reproduction; this is because most individuals chose to produce offspring with people they are unrelated to, thus ‘diluting’ their own genetic pool (3). Because the likelihood of both unrelated individuals sharing the genotype for a recessive allele is rare, there should be no issue. However, when two relatives chose to reproduce, their similar genomes are recombined, thus increasing the likelihood that recessive genes are passed along. Many of these recessive genes encompass massive health consequences for the inbred offspring (1; 2; 4). Inbreeding depression is also known as the reduced biological fitness of a population due to inbreeding. This occurs over several generations of inbreeding. The result is a population that is homozygous for many recessive alleles, and therefore, prone to many health issues. The offspring of inbred populations face a decline in fitness-related survival traits, such as stunted growth rates and infertility (2; 3; 4). This homozygosis, or excessive degree of similarity of alleles in an individual, is the major health consequence of inbreeding. Heterozygosis, on the other hand, gives many animal populations a survival advantage: typically, the more genetic variation among a species population, the greater the advantage for long-term survival and fitness (3; 4). The opposite of inbreeding is known as ‘outbreeding.’ This occurs when two genetically unrelated organisms produce offspring. This occurs much more commonly in nature than inbreeding (1). Health Consequences of Inbreeding In regards to humans and most species, there are many negative health consequences associated with inbreeding. Both genotypically and phenotypically, inbred offspring express many undesirable genetic traits. For example, inbred offspring may (1; 2; 5): * Have reduced fertility or be completely infertile * Have an increased risk for inheriting genetic disorders * Have higher rates of infant mortality * Have higher rates of spontaneous abortions and other pregnancy-associated pathologies * Have poorly functioning immune systems * Exhibit poor growth and development (both physically and cognitively) * Exhibit facial asymmetry * Have a higher risk of being mentally retarded or of very low cognitive ability Coefficient of Inbreeding Equations have been designed to determine the amount of genetic relation between two individuals. “f,” also known as the coefficient of inbreeding, can be determined through an assortment of equations. While many other numerical factors come in to play to alter the outcome, the most basic determination of genetic relation is written as F(A). F(A) is the probability that both alleles in one locus within individual A’s genome are derived from the same gene. This may also be written as F(A,B), or the probability that an allele from individual A and an allele from individual B are identical. By using these equations (among several others), scientists can determine the extent of homozygosity between two individuals (3; 4; 6; 9). “f” is typically determined to be a number between 0 and 1. The closer “f” is to 0, the lower the probability of identical genes; the closer to 1, the higher the probability of identical genes. For example, in regards to a given gene that is both equally dominant and recessive in its inheritance, the expected probability for a random individual would be 0, or 50% (half AA, half aa); the expected probability for an inbred individual would be 1, of 100% (entirely AA or entirely aa) (3; 4; 6; 9). Inbreeding Throughout History Throughout the course of history, inbreeding has been practiced quite commonly. Inter-nobility marriage was commonplace among royal families to preserve royal bloodlines. Ancient Egyptians believed women carried along the family bloodline, so sisters were commonly married to brothers to preserve blood purity. Many examples of European royalty and Asian royalty inbreeding can be traced back to several hundred years ago (2). Some notable examples of cousin-cousin marriages include (8): * Charles Darwin married Emma Wedgewood, his first cousin. * Albert Einstein married both his first cousin and second cousin. * Saddam Hussein married Sajida Talfah, his first cousin. * Thomas Jefferson married Martha Wayles, his third cousin. * Mao Zedong married Luo Yixiu, his second cousin. * Christopher Robin Milne married Lesley Selincourt, his first cousin. Notable Examples of Inbreeding In certain human populations, inbreeding is alive and well to this day. Reformed Mormon communities present in Utah and Arizona in the United States practice inbreeding. Many of the individuals in these Mormon communities pass along Fumarase deficiency, an autosomal recessive disorder affecting the Krebs cycle (1; 2). The Vadoma tribe of western Zimbabe practice inbreeding. The majority of these tribe members pass along the trait of possessing only two toes, known as ectrodactyly. This recessive inheritance has persisted throughout generations because of the tribe’s small gene pool (1; 2). Inbreeding has been discovered in several animal populations around the globe. While many other species may engage in inbreeding, scientists understand inbreeding may be a very different practice in the wild versus in a controlled environment (such as a zoo). Animal species that inbreed include: * Lions * Cheetahs * South American sea lions * Central California sea otters * Certain song sparrow species It has been observed that most mammal populations do not engage in inbreeding (1; 6; 7). References 1) Inbreeding. Wikipedia. http://en.wikipedia.org/wiki/Inbreeding. Updated November 26, 2014. Accessed November 30, 2014. 2) Inbreeding depression. Wikipedia. http://en.wikipedia.org/wiki/Inbreeding_depression. Updated November 21, 2014. Accessed November 30, 2014. 3) Charlesworth D, Charlesworth B. Inbreeding Depression and its Evolutionary Consequences. Ann Rev Ecol Syst. 1987; 18: 237-268. URL: http://www.jstor.org/stable/2097132 4) Charlesworth D, Willis JH. The genetics of inbreeding depression. Nature Rev. November 2009; 10: 783-796. doi: 10.1038/nrg2664 5) Fareed M, Afzal M. Estimating the Inbreeding Depression on Cognitive Behavior: A Population Based Study of Child Cohort. PLoS ONE. October 2014. doi: 10.1371/journal.pone.0109585 6) Vogt D, Swartz HA, Massey J. Inbreeding: Its Meaning, Uses and Effects on Farm Animals. University of Missouri: Department of Animal Sciences. http://extension.missouri.edu/p/G2911. Updated October 1993. Accessed November 30, 2014. 7) Inbreeding in natural populations. Virginia Tech: Department of Fish and Wildlife Conservation. http://fishwild.vt.edu/conservation_genetics/materials/inbre_ta.pdf. Updated Fall 2009. Accessed November 30, 2014. 8) List of coupled cousins. Wikipedia. http://en.wikipedia.org/wiki/List_of_coupled_cousins. Updated November 13, 2014. Accessed November 30, 2014. 9) Coefficient of relationship. Wikipedia. http://en.wikipedia.org/wiki/Coefficient_of_relationship. Updated November 4, 2014. Accessed November 30, 2014.